High voltage level shifting IC with under-ground voltage swing withstanding capability

ABSTRACT

A level shifting circuit provides a reference bias voltage to permit signal transfer between two circuits with different common voltage reference levels. The bias voltage is less than a common voltage reference level for either of the two connected circuits. By providing the bias voltage, the range of variation for the common voltage reference levels between the two circuits is increased when the reference voltages float with respect to each other. The level shifting circuits permit signals to be transferred from a low voltage to a high voltage circuit with increased reliability and noise immunity, and vice versa. The level shifting circuit is particularly useful for driving a half bridge switch configuration, and transmitting a floating current sense signal.

[0001] RELATED APPLICATION

[0002] The present application is based on and claims benefit of U.S. Provisional Application Ser. No. 60/388,583, filed Jun. 12, 2002, entitled HIGH VOLTAGE LEVEL SHIFTING IC WITH UNDER-GROUND VOLTAGE SWING WITHSTANDING CAPABILITY, to which a claim of priority is made.

FIELD OF THE INVENTION

[0003] This invention relates to a circuit for providing an under-ground voltage immune integrated circuit (IC) to be used in industrial applications where gate driver and current sensing devices are often subjected to latch up problems during the under-ground (or negative relative to ground) voltage swings.

BACKGROUND OF THE INVENTION

[0004] In many industrial applications it is necessary to translate a digital signal at TTL or CMOS level from the original ground reference to another reference that is normally floating between much higher voltage limits (0-600V or 0-1200V) with respect to the reference where the signal is generated. On the other hand, a signal generated in the floating stages of the system needs to be translated to the stable ground of the remaining of the circuitry in order to be processed.

[0005]FIG. 1 and FIG. 2 show the solution implemented in a prior art integrated architecture. It is clearly evident that the signal transmission can be done only when the ground shift potential is positive, that is the floating ground voltage is above the logic ground and the V_(DS) on the level shifter MOSFET is high enough to keep the device out of saturation. If the floating ground voltage approaches the logic ground voltage and the MOSFET enters the saturation region, no signal can be transmitted. When the floating ground goes below the logic ground, that is, the ground shift potential is negative, the transmission is not possible at all and even worse, if the positive side of the high side floating supply is below the logic ground level, then other serious problems, such as latch up and consequent fatal destruction may occur in the IC structure depending on its substrate ground connection.

[0006] Summing up, the structures shown in FIG. 1 and FIG. 2 normally transmit correct information only when the positive side of the high side floating supply is at a voltage level at least 4V higher than the logic ground while, on the other hand, the maximum level shift potential is limited to the BV_(DSS) voltage of the high voltage MOSFET normally at 600V or 1200V.

[0007] The MOSFETs in the figures are the only components able to withstand 1200V. The high side referenced circuits are normally low voltage analog or logic circuits surrounded by a high voltage isolation structure created through the insertion of P⁺ and Poly silicon rings.

SUMMARY OF THE INVENTION

[0008] A control or communications signal reference to a low voltage ground is translated for use with a high voltage, high power switching circuit. An input signal is referenced to a floating voltage bias level relative to the input circuit, and the level shifted to the desired voltage range. When used in a half bridge configuration, the translated and level shifted signal is further translated to another high power switching stage that has an additional floating voltage reference. The signals provided for control of the switching circuit are codified into pulses, the duty cycle of which provides the control to the switching circuit.

[0009] In accordance with the present invention, there is provided an integrated circuit with isolation rings to maintain a particular isolation level. For example, the first translation and level shifting stage includes an isolation ring that is capable of withstanding a substrate biasing voltage. The second translation and level shifting stage includes isolation rings to withstand high voltage levels to prevent interference between the first stage or the substrate biasing voltage.

[0010] The present invention permits a signal to be transferred from a low voltage circuit to a high voltage circuit, and vice-versa. Accordingly, the invention may be used to control a high power circuit with low power logic, such as in the case of the control for a half bridge power switching arrangement. Moreover, the invention can be used to read a low power signal with higher power logic, such as in the case of a current sense circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The present invention is described in greater detail below, with reference to the accompanying drawings, in which:

[0012]FIG. 1 is a circuit diagram of a prior art level shifting circuit to transmit signals from a low to high voltage circuit;

[0013]FIG. 2 is a circuit diagram of a prior art level shifting circuit for signals transmitted from a high to a low voltage circuit;

[0014]FIG. 3 is a circuit diagram of a translation and level shifting circuit for signals transmitted from a low to a high voltage circuit according to the present invention;

[0015]FIG. 4 is a circuit diagram of a translation and level shifting circuit for signals from a high to a low voltage circuit according to the present invention;

[0016]FIG. 5 is a schematic diagram of a typical inverter drive for a motor;

[0017]FIG. 6 is a circuit schematic of a conventional gate driver circuit;

[0018]FIG. 7 is a circuit schematic of a gate driver circuit according to the present invention;

[0019]FIG. 8 is a circuit schematic of a conventional current sensing circuit; and

[0020]FIG. 9 is a circuit schematic of a current sensing circuit according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The basic concept of the invention is depicted in FIG. 3 and FIG. 4. The circuits shown permit signal transmission between two circuits whose reference voltage can swing in the range (−)Vbias to (Vswitch−Vbias) where Vswitch is the maximum voltage withstand of the high voltage MOSFETS and (−)Vbias is an external voltage, negative referring to the logic ground, typically of (−)50V or (−)100V.

[0022] Referring to FIG. 3, Low Side to High Side Transmission, the signal to be transmitted is applied to a switch M1 (normally implemented through a P channel MOSFET) that sources current into a receiving device (normally a resistor or a low voltage MOSFET in active load configuration). The receiving device is referenced to the negative bias level and level shifting to a lower voltage reference is thus achieved. This signal is then used to activate another switch (normally implemented through an N channel MOSFET) that finally translates the information to the floating stage of the IC at 600V or 1200V.

[0023] The information cannot be transmitted continuously between circuit 1 and circuit 2 in FIG. 3 because of the excessive power dissipation during the MOSFET's on-state. In fact, though the drain current can be low, the high voltage present across M2 results in high power dissipation in the IC. This is normally avoided by means of a pulse transmission. That is, the information is transmitted though the structure as two pulses, turn on and turn off, and normally codified in the duty cycle of the resulting square wave.

[0024] The same solution is applicable in the idea here described. It should be noted that the two circuits, as well as the two MOSFETs M1 and M2 do not need to be classified at the same isolation level. In particular, MOSFET M1 and all isolation rings around circuit 1 need to be able to withstand only the substrate biasing voltage. If this voltage is 100V, for example, then a 200V silicon structure is enough for isolation purposes. On the other hand, MOSFET M2 and isolation rings around circuit 2 have to be high voltage structures, and have to withstand 600V or 1200V so that they normally occupy much more space in the silicon.

[0025] Summing up, in FIG. 3, the integration onto silicon requires two isolation structures, one for the high-side part of the circuit and the other for the logic ground referenced circuit. While the area loss for the high side portion doesn't change from that of the state of the art, the isolation around circuit 1 and Pch MOS level shifter is added, with an attendant certain area of occupation. As described, however, these last high voltage structures are isolated at much lower voltage ratings, which means they are much smaller then those used in circuit 2 and the overall increase in area does not typically exceed 30%.

[0026]FIG. 4 shows the invention as applied to transmission from High Side to Low Side. Its operation is similar to that of FIG. 3, but in this case MOSFET M1 and circuit 2 needs to be created with high voltage isolation capability while MOSFET M2 and circuit 1 just need to withstand the negative biasing voltage.

[0027] A further pin is needed for the negative biasing on the IC package frame. This pin will be polarized at some tens volts difference (−50V or −100V usually) compared to the IC ground level. This usually requires attention to minimum clearances on the pin out design of the package, similar to, but less restrictive than used for the high side portion of the device.

[0028] In the field of electric motor drives the most used architecture for the power stage is a 3-phase Voltage Source Inverter (“VSI”). The VSI structure is used to convert a DC input voltage to an AC output, which lets the user control voltage and frequency of the three-phase voltage applied to the motor thus varying torque and speed. The same type of schematic, often in H bridge configuration, is used in UPS circuits and in Power Supply circuits in general and the invention here described is generally applicable.

[0029] A three-phase inverter is made of 6 static switches (IGBTs or MOSFETs typically) as shown in FIG. 5.

[0030] 1. The Gate Drivers

[0031] The static switches of FIG. 5 need to be driven by a voltage applied to the gate-emitter terminals of each switch, according to the modulation strategy used in the application. The emitters of all high side switches are connected to the output phase so that a floating gate-driver is needed, that follows the variation of the output voltage and continues to apply the gate-emitter voltage, on and off, according to the PWM sequence. Also the low-side emitter voltage changes considerably across the DC bus minus voltage during normal system operation, because of the presence of parasitic elements on the power connections, in particular resistive and inductive effects, that cause the low side emitter voltage swing.

[0032] In applications involving more than I kilowatt of power, these voltage swings due to the parasitic elements present in the system layout are not negligible; their magnitude can easily reach and exceed the gate-emitter command voltage and heavily affect the overall functionality and life to the application. This effect is even worse in short circuit situations, when the current flowing in the parasitic inductances is very high and the Lenz effect is strong. Accordingly, even the low-side gate-driver needs to be floating, though in a lower voltage range.

[0033] Taking an example from an industrial motor driver supplied at 550 Vdc and with 50 amp rated IGBTs, the emitter voltage of the high-side switch can vary in the range of −50V to +1000V during short circuit withstand. For the low-side emitters the voltage range could be from (−)50V to +100V depending on the value of parasitic parameters in the power path's layout. These values can be read when using, as a reference, the emitter of another IGBT in a leg not carrying the short circuit current.

[0034]FIG. 6 is a simplified block schematic of a typical integrated half bridge gate-driver (1 high-side and 1 low-side driver for one leg only) with a prior art type of transmission structure. Pin Out Definitions of FIG. 6: Vss: logic ground Vdd: logic supply HIN: high-side input command LIN: low-side input command COM: low-side power ground Vcc: low-side driver supply LO: low-side driver output Vs: high-side driver floating ground Vb: high-side driver floating supply HI: high-side driver output

[0035] Looking at the driver block schematic of FIG. 6, the following observations can be made:

[0036] When the voltage Vb is less than that of COM, the transmission of command from HIN to HI becomes impossible;

[0037] When the voltage level of COM becomes higher than that of Vdd the transmission from LIN to LO becomes impossible;

[0038] When Vb, which is also the N-Epi floating pocket potential, is less than the substrate voltage (either COM or Vss, depending on the IC layout), either the “Substrate-floating-Epi” diode goes into direct conduction and latch up or general failure may occur in the IC structure.

[0039] The present invention permits the correct information transmission from control circuit to IGBTs when the relative emitter voltage is negative, increasing the reliability of transmission, in particular during short circuit withstands, and completely avoids latch up problems.

[0040]FIG. 7 is an example of a gate-driver using the present invention. Pin Out Definitions of FIG. 7: Vss: logic ground Vdd: logic supply HIN: high-side input command LIN: low-side input command COM: low-side power ground Vcc: low-side driver supply LO: low-side driver output Vs: high-side driver floating ground Vb: high-side driver floating supply HI: high-side driver output Vbias: Negative bias voltage input

[0041] In FIG. 7, the three separated epi-pockets are indicated (using different shading) to illustrate the different isolation voltage structures possible.

[0042] In particular, N-Epi Pocket 1 and N-Epi Pocket 2 have to withstand only the Vbias voltage. These pockets are realized in the silicon using high voltage isolation rings adequate for this purpose; normally 100V or 200V are enough. Also all level shifting MOSFETs M1 and M2 have the same BVDss.

[0043] The N-Epi pocket 3 is the very high voltage side of the driver and is normally isolated at 600V or 1200V. In this case, the structure for MOSFETs M3 are at the same voltage of 600V or 1200V.

[0044] A further supply and a dedicated pin for the Vbias voltage is provided. The Vbias pin is polarized much lower than both Vss and COM voltage levels. The magnitude of this negative Vbias voltage will then be the immunity margin for the correct functioning of the IC when the system shows under-ground voltage swings.

[0045] 2. The Current-Sense Devices.

[0046] The same problem previously described arises during current sensing in the output phases of the inverter. The high-side part of the circuit is connected to the output phase and senses the motor phase current measuring the drop across an external shunt resistor. The information is then transformed in a burst of variable length pulses or a square wave with variable duty cycle, and translated to the low-side part of the IC in order to be processed and then provided to an external processor. Using the present invention, the pulse to transmit can be distorted or even canceled if the ground shift potential becomes negative during the transmission interval, resulting in a high error rate for the related codified information.

[0047]FIG. 8 shows an example of an integrated current sense device with a prior art transmission structure.

[0048] Problems related to the loss of information previously described disappear when using the solution proposed by this invention. A simplified block schematic using the invention in a current sense application is shown in FIG. 9. Level shifting MOSFET M1 is rated at a much lower isolation voltage than M2. This solution does not increase excessively the transmission delay between the two parts of the IC and the reduction effect on the transmission throughput is negligible.

[0049] In FIG. 9, the two N-Epi pockets at different levels of isolation are shown in different shading. N-Epi 1 and FET M1 can be isolated at 100V or 200V while N-Epi 2 and Fet M2 are the very high voltage isolation structures.

[0050] A new idea to solve the common problem of under-ground voltage swings in integrated circuits used in industrial applications has been presented. This new idea not only solves problems related to loss of communication when under-ground swings occur, but also overcomes problems created by the intrinsic nature of the ICs having a substrate that can cause IC latch-up when one of the N-Epi wells, normally in inverse polarization, occasionally goes into direct conduction.

[0051] Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein. 

What is claimed is:
 1. A circuit for transferring an input signal between differently referenced circuits, comprising: an input stage circuit referenced to a first common voltage reference and operable to receive the input signal; a bias voltage coupled to the input stage for providing a shifted common voltage reference; a level shifting circuit coupled to the input stage and the bias voltage for translating the input signal to be referenced to the bias voltage; an output stage circuit referenced to a second common voltage reference and coupled to the level shifting circuit to receive the translated input signal to provide an output related to the input signal and referenced to the second common voltage reference.
 2. The circuit according to claim 1, further comprising: a second level shifting circuit coupled to the input stage for translating the input signal to be referenced to a third common voltage reference; and a second output stage coupled to the second level shifting circuit and referenced to that third common voltage reference to provide a second output referenced to the third common voltage reference.
 3. The circuit according to claim 1, wherein the input circuit includes a switch referenced to the bias voltage and operable based on the input signal.
 4. The circuit according to claim 3, further comprising a pulse generator in the input stage and coupled to the input signal; and the pulse generator operable to output pulses to the level shifting circuit with a pulse duration related to a value of the input signal.
 5. The circuit according to claim 2, wherein the first and second output stages are low and high drivers for half bridge switching circuit, respectively.
 6. A circuit according to claim 1, wherein the circuit is implemented in an integrated circuit.
 7. The circuit according to claim 6, further comprising a voltage isolation structure around the input stage capable of withstanding a voltage level outside a range of that in the input stage.
 8. The circuit according to claim 6, further comprising an isolation structure around the output stage for withstanding voltage levels outside a range of those in the output stage.
 9. A level shifting reference translation circuit integrated on a substrate, comprising: a logic output circuit referenced to logic ground for providing a logic level output; a biasing voltage coupled to the logic ground reference and the substrate to bias the substrate with respect to the logic ground reference; a high voltage input circuit referenced to a high voltage ground reference level for providing a high voltage input; a level shifting circuit coupled to the input circuit and the output circuit and referenced to the biasing voltage for transferring a signal from the high voltage input to the logic ground referenced output.
 10. The circuit according to claim 9, wherein the biasing voltage produces a negative voltage bias on the substrate with respect to the logic ground.
 11. The circuit according to claim 9, further comprising an isolation structure for the output circuit, the isolation structure being able to withstand a outside a range of that of the voltage in the output circuit.
 12. The circuit according to claim 9, further comprising a voltage isolation structure around the output stage sufficient to withstand voltages outside a range of those in the output stage.
 13. A current sense circuit for transferring a current sense signal between two circuits with different voltage references, the current sense circuit comprising: an input circuit for receiving a current sense signal referenced to a first common voltage reference level; a reference level transfer circuit for transferring the current sense signal reference to another reference; a bias voltage coupled to the transfer circuit to permit the current sense signal to propagate through the transfer circuit; an output circuit coupled to the transfer circuit and operable to produce a current sense output signal with a shifted voltage level and a second common voltage reference level.
 14. The circuit according to claim 13, wherein the input circuit, the transfer circuit and the output circuit are constructed on a single integrated circuit.
 15. The circuit according to claim 13, wherein the bias voltage is less than the first common reference voltage level.
 16. The circuit according to claim 13, further comprising a voltage isolation structure around the output circuit being capable of withstanding a voltage level outside that of a voltage range in the output circuit.
 17. The circuit according to claim 13, further comprising a voltage isolation structure around the input circuit and capable of withstanding a voltage outside that of a voltage range in the input circuit.
 18. A method for transferring a signal between two circuit with different common voltage references, comprising: receiving an input signal to an input circuit referenced to a first common voltage reference; providing a bias voltage to the input circuit as an intermediate voltage reference level; shifting the input signal voltage reference level to the bias voltage reference; transferring the signal to an output circuit having a second voltage reference level such that the signal is referenced to the second voltage reference level; and outputting the signal from the output circuit.
 19. The method according to claim 18, further comprising: transferring the bias voltage referenced signal to a second output circuit referenced to a second output voltage level, such that the transferred signal is referenced to the second output voltage reference level; and outputting the signal reference to the second output voltage reference level.
 20. The method according to claim 18, further comprising isolating the input circuit from voltage levels outside the range of voltage levels in the input circuit.
 21. The method according to claim 18, further comprising isolating the output circuit from voltage levels outside the range of voltage within the output circuit.
 22. The circuit according to claim 19, further comprising isolating the second output circuit from voltage levels outside the range of voltages within the second output circuit.
 23. A method for transferring a signal between two circuits with different common voltage reference levels, comprising: receiving an input signal to an input circuit referenced to a first common voltage reference level; shifting the signal voltage reference level to be referenced to a biasing voltage; transferring the biasing voltage referenced signal to an output circuit having a second common voltage reference level, such that the signal is referenced to the second common voltage reference level; and outputting the signal from the output circuit with the second common voltage reference level.
 24. The method according to claim 23, further comprising codifying the input signal as a length of a pulse transferred between the input circuit and the output circuit.
 25. The method according to claim 23, further comprising supplying the biasing voltage as a voltage level lower than the first or second common voltage reference levels.
 26. The method according to claim 23, further comprising transferring a current sense signal from the input circuit to the output circuit.
 27. The method according to claim 23, further comprising isolating the input circuit from voltages outside a range of voltages within the input circuit.
 28. The circuit according to claim 23, further comprising isolating the output circuit from voltages outside a range of voltages within the output circuit. 